Projectile rotating band



March 15, .1960 w. A. ZISMAN ET AL 2,928,348

PROJECTILE ROTATING BAND Filed Jan. 7, 1955 SINTERED POLYMER WASHER 4STEEL JACK ET SINTERED POLYMER FILM SI NTERED POLYMER FILM IELELEWILLIAM A. ZISMAN VINCENT G. FITZSIMMONS SINTERED POLYMER INVENTORjhired States PROJECTILE ROTATING BAND William A. Zisrnan, Silver Spring,Md.,' and Vincent G. Fitzsimmons, Washington, D.C., assignors to theUnited States of America as represented by the Secretary of the Navy andused by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

This invention relates to ammunition, more particularly to animprovement in projectiles which is designed to increase the firing lifeof gun barrels.

Metal fouling, if unchecked, will eventually terminate the firing lifeof gun barrels. It is caused'by the deposition of softer metal from theprojectiles on to the rifiing of the gun during firing and byaccumulation. produces a roughened surface on the rifiing which isreflected in deviations in the spin imparted to the projectile andthereby loss in accuracy in the gun fire. Loss in muzzle velocity of theprojectile may also result from metal fouling. To overcome this metalfouling, the earlier jacket of ciipro-nickel on bullets and rotatingband of soft copper on artillery type projectiles have been generallyreplaced by those made of the harder, but still neces-' sarilyrelatively soft, gilding metal. For single [shot weapons, this measurehas all but eliminated metal fouling. It has not, however, met withequal success in the case of rapid fire guns, especially those which areautomatic, where metal fouling is still a problem.

We have found that the firing life of gun barrels of all calibers can bematerially increased by so modifying the projectile that the portion ofits surface which normally makes contact with the rifiing of the gun onfiring is at least in part essentially sintered polytetrafiuoroethylene.Polytetrafiuoroethylene is a polymer known in the trade as Teflon. By somodifying the projectile, there eventuates, on firing of the gun, asystem of contact between the projectile and the rifling which is notmetal-to-metal but polymer-to-polymer. This flows from the fact thatsintered polymer is transferred from the projectile to the V rifiing byrubbing to form on the latter a thin coating or film of the sinteredpolymer. The coating on the rifiing does not build up by accumulation toan undesirable degree of thickness due to the non-self-adhering propertyof the sintered polymer, the thickness of the coating being that of thethin film as initially applied by the transfer from the projectile. Whenbreaks occur in the film, renewal of the open portions is effected inthe same manner; the exposed metal portions of the rifling receivingsintered polymer by transfer from the subsequently fired projectile; Areverse arrangement in which the rifiing is coated with sintered polymerand the projectiles are of the usual construction is impractical sincemovement of the projectile over' the riding will remove portions of thecoating until eventually it is worn off.

Among the beneficial results flowing from the presence of the sinteredpolymer on or as that portion of the surface of the projectile whichcontacts the rifling of the gun, is the absence of metal fouling.Another beneficial result is reduction in the rate of wear of the gunbarrel by friction, the sintered polymer having a low coefficient offriction. A further beneficial result and one of importance to rapidfire guns, especially those which are atent automatic, is reduction inthe occurrence of heat erosion of the gun barrel. The sintered polymerhas a low thermal conductivity and this property in the polymer film onthe riding serves-to delay transfer of heat from the hot, hightemperature gases to the metal of the surface of the gun bore and byconsequence, promotes discharge of a greater portion of the heat fromthe gun barrel by way of the muzzle. Another beneficial result is agreater degree of freedom from firing debris in the gun barrel, thesintered polymer surface on the projectile having the remarkableproperty of cleaning the gun barrel. Still another beneficial result isa tendency to give a greater muzzle velocity for the projectile. Yetanother beneficial result is elimination of the need of oiling theprojectile with consequent avoidance of pick-up of dust and sand by theoil.

Modification of the projectile in accordance with the present inventionmay be done under several forms depending upon the type of projectile.The most general of the forms is a thin film of the sintered polymerover the surface of the projectile including that of the rotating bandin the case of artillery type projectiles. A particular form ofmodification is employed in the case of projectiles of a metal of lowmelting point, e.g., a lead bullet, since a coating of the sinteredpolymer cannot be successfuly applied thereto. For such projectiles apreformed insert of the sintered polymer is made an integral part of theprojectile. In the case of artillery type projectiles modification ofthe projectile can be also effected j by the provision of a rotatingband which in'its construction embodies in the periphery thereof asurface which is essentially sintered polytetrafluoroethylene.Modification of the projectile in the several ways described above isillustrated in the accompanying drawing which forms part of thedescription of the present invention. The thickness'of the sinteredpolymer coating and the reach of the rotating band beyond the peripheryof the body of the projectile has been exaggerated in the drawing ineach instance for purposes of illustration.

In the drawing:

Figure 1 shows a small arms projectile of low melting point metalmodified in accordance with the invention through the inclusion of aninsert of sintered polytetrafluoroethylene, the periphery of which formspart of the surface of the projectile,

Figure 2 shows a small arms projectile of high melting point metalmodified in accordance with the invention by the presence thereon of athin film of the sintered polymer,

Figure 3 shows an artillery type projectile modified in accordance withthe invention by the presence thereon, including the rotating band, of athin film of the sintered polymer, v

Figure 4 shows an artillery type projectile, partly broken away,modified under another form in accordance with the invention through theprovision of a rotating band of a new construction which containssintered polytetrafluoroethylene as part of its periphery,

Figure 5 is a view in detail of a section of the new rotating band shownon the projectile of Figure 4,

Figure 6 is a view, partly in cross-section, of another new rotatingband for the modification of artillery type projectiles in accordancewith the invention, the new band also containing sinteredpolytetrafluoroethylene as part of its periphery, and

Figure 7 shows yet another new rotating band forthe modification ofartillery type projectiles in accordance with the invention, this newband being of the type shown in Figure 4.

Referring to Figure l, the projectile 1 of low melting point metal is alead bullet which contains as anintegral part thereof, a washer 2 ofsintered polytetrafiuoroethylene. A coating of the sintered polymercannot be successfully applied to low melting point metals because ofthe high temperature necessary for sintering of the polymer. The washer2 is located in the bullet just behind the ogive and is conterminouswith the adjacent or bearing surface of the bullet. A suitable thicknessof the sintered polymer washer 2 is on the order of .025 inch for allsmall arms bullets, regardless of caliber. As illustrated, the compositebullet oi the invention is for a .30 caliber rifle.

The new composite bullets may be made by casting in bullet molds in theusual way with the modification that the washer 2 of the sinteredpolymer is previously placed in the predetermined position in the moldcavity. The hole in the washer 2 of sintered polymer should be largeenough to allow the molten, low melting point metal, such as lead or oneof the usual lead alloys, to flow through and fill the bottom as well asthe upper portion of the bullet mold cavity, whereby on cooling thecasting embraces the washer of sintered polymer as an integral part ofthe bullet. The washer 2 can be formed by cutting from a tape of thesintered polymer of the required thickness, the outer diameter of thewasher being that of the bullet. The Washer is dropped into the moldcavity to lie in a horizontal position and is located at the properlevel by virtue of the convergence of the Walls of the mold cavity whichare tapered to form the ogive of the bullet.

In Figure 2 the projectile is a steel or steel-jacketed bullet 3 whichfor purposes of illustration is shown as for .30 caliber rifieammunition. The bullet is provided with the usual cannelure i forcrimping of the cartridge case thereto. A thin film 5 of sinteredpolymer is coated on the steel surface of the bullet which, as shown,preferably extends back only to the forward edge of the cannelure. Thislimitation on the area covered by the sintered polymer coating ispreferred in order to avoid any difficulties which the coating mightpose to effective crimping of the cartridge case to the bullet. Thethickness of the sintered polymer film 5 is on the order of 0.6 milwhich is a practical value and sufiicient to provide a thin coating ofthe sintered polymer on the rifting of the gun as the bullet passesthrough the gun bore. Bullets of various calibers made of or jacketedwith a high melting point metal may be advantageously modified in thismanner.

Figure 3 shows an artillery type projectile modified in accordance withthe invention and in the manner described above for steel andsteel-jacketed bullets. The sintered polymer coating in the case ofartillery type projectiles extends over the rotating band. This isessential. For purposes of illustration, the projectile is shown as for20 mm. ammunition used in automatic rapid fire guns. It is ofconventional construction and arrangement and has a steel shell 6provided with a cannelure '7, a threaded cap section and a rotating band8 of gilding metal. The thickness of the sintered polymer film 9 is onthe order of 0.6 mil which is also a practical value for the coating onartillery type projectiles of all calibers.

In Figure 4, the artillery type projectile is modified in accordancewith the invention thorugh a new rotating band. The projectile 11 is thesame as that shown in Figure 3, the shell being made of steel and havinga cannelure 12. The rotating band 13 is formed of a threedimensional,helically woven wire mesh which has been filled with sinteredpolytetrafluoroethylene. The detail of a section of the new rotatingband shown in Figure 5 depicts the open weave of the wires 14 and therandom covering of the same by the sintered polymer 15 which fills theopenings in the metal mesh up to the level of the wires 14. As theprojectile is moved through the gun bore, the compressive action of themoving projectile on the rotating band 13 causes transfer of sinteredpolymer from the band to the rifiing of the gun and To assist insecuring the polymer filled mesh band 13 to the body of the projectile,the seat of the retaining groove for the band is roughened in a suitablemanner as, for example, by milling, to provide a'better grip for themesh structure of the band. The dimensions for the band and depth of theretaining groove follow usual practice. The rotating band for a 20 mm.projectile, for example, has a diameter of about 21 mm. and a width ofabout one-quarter inch, and the retaining groove in the body of theprojectile has a depth of about threesixteenths inch.

Suitable stock for making of the rotating bands 13 is a tubular,three-dimensional, helically woven, ductile wire mesh having a densityof about 60 to and a pore size of about 10 microns or less. The weave ofthe tubular mesh stock is such that grainwise, the angle ofcrossing ofthe wires is less than The Wires are welded to each other at theircrossings which can be accomplished by sintering to effect self-welding.This structure in the mesh gives a rotating band which has low shear inthe direction of the longitudinal axis of the gun bore and maximumtensile strength in the direction perpendicular to the rifiing of thegun. The low shear along the longitudinal axis allows the band to yieldeasily in the direction of movement of the projectile over the rifiing,whereas the greater tensile strength along the perpendicular axis forcesconformity of the surface of the band to the contour of the rifiing andaffords strong resistance to disintegration of the band by the highcentrifugal force exerted in the rotation of the projectile. Tubularmesh stock of the aforesaid kind in which the wire is made of pure ironis available on the market under the trade name Poroloy and used forfilter work. Other ductile metals from which the wire may be made are,for example, copper, brass and bronze. The rotating bands are cut fromtubular mesh stock which is slightly larger in both inner and outerdiameter than those of the retaining groove in the projectile. In width,the bands are cut slightly smaller than the width of the retaininggroove. Filling of the mesh bands with sintered polymer can be carriedout by placing them in a housing provided with an inlet through which ispumped a thick aqueous suspension of polytetrafluoroethylene, forexample, of 85-95% solids concentration, which passes into the meshbands and is collected therein, and with an outlet for the efiiuentwater. The polymer particles agglomerate in the wire mesh to promotecollection therein. The inflow of the suspension is carried on until bythe build up of the resistance to the inflow indication is received ofthe filled-up condition of the bands. Excess polymer is wiped from thesurface of the bands in appropriate manner. Excess water is removed tothe extent of an air-dried condition of the polymer which can beaccomplished by allowing the filled bands to stand at room temperaturefor several hours. The air-dried filled bands are then heated to effectsintering of the polymer and cooled to form the finished article. Thefinished band is applied to the projectile by swaging, in whichoperation the band is compressed to size with flow of the metal meshlaterally to fill the retaining groove in the projectile.

In Figure 6 the new rotating band 16 is of laminar construction in whicha pair of metal washers 17 have a matching washer 18 of sinteredpolytetrafiuoroethylene sandwiched therebetween. The diameter and widthof the new laminated rotating band and the depth of the retaining groovetherefor as well as the arrangement and attachment of the new band tothebody of the projectile follow usual practice in the art. For a 20 mm.projectile, for example, the width of the new rotating band '16 is aboutone-quarter inch, the diameter thereof about 21 mm. and the depth of theretaining groove about threesixteenths inch. The washers 17 are made ofa ductile metal, such as copper or brass, which latter maybe gildi iting metal, and in width are each about four and one-half times that ofthe sintered polymer washer 18. The thickness of the sintered polymerwasher for the rotating band of a20 mm. projectile, for example, is onthe order of twenty-five thousandths inch which magnitude of thicknessis sufiicient, also, in, the case of the rotating band for 37 and 40 mm.projectiles to provide a coating of the sintered polymer on "the riflingof the gun. The washer 18 can bestamped from a tapeof the sinteredpolymer of the required thickness, the outer diameter of the washerbeing that of the metal washers 17.} i v,

Construction of the new rotating band 16"is accomplished by coating oneside of each of the metal washers 17 with a thin film of an aqueoussuspension of polytetrafluoroethylene, on the order of 0.2-0.3 milthick, bringing the film to the air-dried condition, forming a sandwichassembly of the coated metal washers 17 with the sintered polymer washer18 aligned therebetween and in contact with the air-dried film on themetal washers, and while holding the parts in aligned relationship bymeans of the application of a light pressure, for example, in a platentype press, heating the assembly to a temperature sufficient to sinterthe air-dried polymer film on the metal washers 17 and soften thesintered polymer washer 18,

and withdrawing the assembly and allowing it 'to coolto set thecementing between the parts.

Figure 7 shows a mesh type rotating band 19 similar in construction tothat of Figures 4 and 5 but modified to include a collar 20 which servesto facilitate swaging of the band onto the projectile and to-protect themesh against permanent deformation in the swaging operation. The collar20 and the mesh 21 are made of the like ductile metal, for example,brass, and the mesh mounted on the metal collar by a self-welding ofcontiguous portions of the wire to the collar. This composite mesh bandis made by helically weaving the ductile wire on a tube of the ductilemetal serving as the mandrel. The ends of the woven mesh are closed byflame heating to cause self-welding of the wires and the structureheated to cause sintering of adjacent wires of the mesh to the metaltube and to each other at their'crossings. The mesh portion of the bandis filled with sintered polymer Filling of the mesh with polymer can becarried out by placing a rubber sleeve over the tubular structure andthrough openings in the rubber sleeve, one to each side of the centerand aisubstantial distance toward the respective end, forcing a streamof the thick aqueous suspension of the polymer into the mesh until it isfilled. A centrally located hole in the rubber sleeve provides for outflow of the water and excess polymer suspension. After removal of excesspolymer from its surface, the filled mesh structure is subjected to airdrying of the polymer and sintering of the same. The rotating bands arecut from the finished tubular stock to a width slightly less than thatof the retaining groove in the projectile and swaged into the groovewhich is milled at the bottom to promote retention of the band on theprojectile.

Polymer suspensions suitable for use in modifying projectiles inaccordance with the invention are-low viscosity dispersions of colloidalpolytetrafiuoroethylene in essentially water as the suspending medium.The particle size of the polymer may be on the order of 0.1 micron. Asmall amount of a wetting agent is present in the suspension to promoteinitial dispersion of the polymer particles in the water and to maintaintheir dispersed condition. In addition to the polymer, the suspensionmay contain pigments, notably such as contribute to increase inprotection against moisture corrosion of the metal. A preferred aqueouspolymer dispersion for purposes of the invention is one containingchromic acid, which pigment affords a strong degree of protection toiron and steel surfaces against rusting, especially when the sinteredpolymer coating thereon is formed of at least two layers of thesuspension which have been separately applied and sintered. The secondlayer of sintered polymer tends to seal-off pinholes in the first layerand thereby reduce the incidence of occurrence of moisture at thesurfaces of the metal. Aqueous polymer suspensions to be used for primeror single layer coatings on iron, steel, brass and copper are highlyacid. The acidity can be provided by the addition to the polymersuspension of an appropriate acid, for example, phosphoric acid, and inpart by the use of an acid-reacting wetting agent for dispersion of thepolymer particles in the water. Aqueous polymer suspensions suitable foreffecting modification in projectiles in accordance with the inventionare available commercially, or described in one or more of the followingUS. Patents: 2,478,229, 2,562,117 and 2,562,118. r

The surface of the projectiles, rotating band, wire mesh or metal platesof the laminated type of new rotating band, as the case may he, shouldbe in a clean condition before application of the aqueous polymerdispersion since dust, oil and grease interfere with the adhesion ofthe. polymer suspension to the metal. Any suitable method may be usedfor this purpose. In some instances,

'such as in the case of surface of the body of the projectile, washingwith unleaded gasoline or other aliphatic hydrocarbon degreasing solventmay be suflicient to clean the surface. Non-ferrous metal surfaces maybe cleaned by washing them in a hot water solution of 3% each by weightof trisodium phosphate and Aerosol '01 (sodium dioctylsulfosuccinate)after which they are thoroughly rinsed in water and air dried. Asatisfactory method for cleaning ferrous metal surfaces is to subjectthem to a light sand-blasting using a very finegrit, for example, No.silica sand, followed by washing with a volatile aliphatic hydrocarbondegreasing solvent. Handling of the cleaned metal surfaces should beavoided until the wet-coating of the polymer suspension thereon hasbecome air dried since finger marks on the metal surface preventadhesion of the polymer suspension thereto. Oxide film on the metal neednot be removed and, in fact, should be retained since it promotesadhesion of the polymer suspension to the metal.

Coating of the metal rotating bands and,where practicable, of the bodyof the projectile can be done by dipping or spraying, preferably,however, by spraying which provides better control both in respect tothickness and the evenness of the coating. Spray viscosity in theaqueous polymer suspensions can be had at solids concentration of 50% orless. Small proportions of toluene emulsified in the suspensions improvetheir flow quality for spraying. The thickness of the coatings can bevaried by varying the solids concentration of the suspensions, the lowerthe solids concentration therein, the thinner the coated layer. Thinningof the polymer suspensions can be done by dilution with distilled water.The sintered polymer has low strength and to improve the adhesionthereof and the availability of the same as a dry lubricant for therifiing of the gun, the surface of the rotating band is roughened, suchas by grooving or knurling, before applying the aqueous suspension ofthe polymer thereto.

Concentration of aqueous polymer suspensions for the purpose ofobtaining high solids content suspensions for the filling of wire meshtype rotating bands can be accomplished by freezing the polymersuspension, thawing them and during the thawing removing the water bydecantation or any other way found suitable to leave an aqueous pastehaving a solids content on the order of to 7 It is essential that excesswater be removed from the wet polymer coatings on the projectile orrotating band, etc., before sintering in order to avoid blistering orvoid formation in the sintered coatings. The coatings are thereforebrought to air dried condition before the sintering. To accomplish this,the coatings are allowed to stand at room temperature until air dried,for which genenemas;

erally about two to three hours will sufiice depending upon the humidityof the atmosphere. Some acceleration of the drying may be practicedbysubjecting the Wet coated articles to warm air at a temperature below100 F. in an oven of the convection heating-recirculating air type.

Sintering of the air dried polymer present as a coating on theprojectile or on the rotating band or on both, or as the filling in thewire mesh type band or as the adhesive on the metal Washers of thelaminated type band is carried out in accordance with known practice byheat-- ing the articles to a temperature between about 675 and 750 F.,preferably at about 700 F. The heating may suitably be carried out in anelectrically heated oven of the air recirculating type. The timerequired for the sintering is almost entirely determined by thatnecessary to bring the metal of the articles to temperature, since theactual sintering of the polymer takes place within a matter of a fewseconds once the metal reaches sintering temperature. The occurrence ofsintering can be visually determined by observing a Wet condition of thepolymer surface and coalescence of the incipiently fused polymerparticles to a continuous mass.

Since the invention described herein may be variously practiced withoutdeparting from the spirit and scope thereof, it is not intended that itbe limited except as is required by the appended claims.

What is claimed is:

1. A rotating band for projectiles comprising a ring of porous,three-dimensional, helically Woven ductile wire mesh having an angle ofwire-crossing, grainwise, of less than 90, the ductile wires welded toeach other at their crossings, a density of from about 60 to 85%, a poresize of about microns and less and filled with essentially sinteredpolytetrafluoroethylene.

'2, A rotating'band for projectiles comprising a ring of porous,three-dimensional, helically woven ductile wire mesh having an angle ofwire-crossing, grainwise, of less than 90, the ductile wires welded toeach other at their crossings, a density of from about to a pore size ofabout 10 microns and less, welded at adjacent portions to a ductilemetal collar and filled with essentially sinteredpolytetrafluoroethylene.

3. A projectile provided with a rotating band thereon. which comprises aring of porous, three-dimensional, helically woven ductile wire meshhaving an angle of wire-crossing, grainwise, of less than the ductilewires welded to each other at their crossings, a density of from about60 to 85, a pore size of about 10 microns and less and filled withessentially sintered polytetratluoroethylene.

References Cited in the file of this patent UNITED STATES PATENTS648,243 Dawson Apr. 24, 1900 707,135 Maxim Aug. 19, 1902 1,944,884Gerlich Ian. 30, 1934 2,360,473 Calkins Oct. 17, 1944 2,478,229 BerryAug. 9, 1949 2,562,118 Osdal July 24, 1951 2,663,259 Catlin Dec. 22,1953 2,689,380 Tait Sept. 21, 1954 2,715,617 White Aug. 16, 19552,731,068 Richards Jan. 17, 1956 2,809,587 Musser Oct. 15, 1957 FOREIGNPATENTS 586,674 Great Britain Mar. 27, 1947 657,085 Great Britain Sept.12, 1951 707,065 Great Britain Apr. 14, 1954

3. A PROJECTILE PROVIDED WITH A ROTATING BAND THEREON WHICH COMPRISES ARING OF POROUS, THREE-DIMENSIONAL, HELICALLY WOVEN DUCTILE WIRE MECHHAVING AN ANGLE OF WIRE CROSSING, GRAINWISE, OF LESS THEN 90*, THEDUCTILE WIRES WELDED TO EACH OTHER AT THEIR CROSSINGS, A DENSITY OF FROMABOUT 60 TO 85, A PORE SIZE OF ABOUT 10 MICRONS AND LESS AND FILED WITHESSENTIALLY SINTERED POLYTETRAFLUOROETHYLENE.